Conventionally, gas turbine engine oil systems have a continuous oil flow which feeds to bearings, gears and splines. Also, components of a gas turbine engine that are manufactured from non-corrosion resistant materials are provided with oil to prevent corrosion. Most gas turbine engines use a self-contained recirculatory lubrication system in which the oil is distributed around the engine and returned to an oil tank by pumps. However, some engines use what is known as a total loss or expendable system in which the oil, after lubricating the engine, is either burnt in the engine's normal combustion process or is allowed to spill overboard.
In known arrangements, oil is supplied to desired locations on the basis of that needed during take-off conditions where the performance of the engine is at its maximum. However, a gas turbine engine operates for a large proportion of its cycle at cruise conditions, which requires less performance and hence a lower supply of oil to its components than at take-off.
It is known to have a lubrication system of this type where the maximum oil flow is not supplied to the engine components all the time. In JP 58051229, for example, a system is described in which a solenoid valve permits a proportion of the oil flow from the pump to be recirculated to the pump inlet, when the engine is running at low-power. Recirculating systems such as this have the disadvantage that the pump is operating at full power all the time, which increases power consumption, noise and oil heating.
U.S. Pat. No. 4,245,465 discloses a three-stage poppet valve to modulate the oil flow in response to engine speed. U.S. Pat. No. 5,180,034 discloses a system with a variable-speed oil pump, controlled in response to engine speed, throttle setting and other parameters, to vary the oil flow to the engine components.
Systems that modulate the flow of oil in this way have the disadvantage that, where oil is directed to a specific region within a component (for example, within a bearing chamber) by means of a targetted jet or nozzle, the jet of oil will only be aimed correctly for one flow condition, and when the flow is modulated by the control system to a different condition, the aim of the oil jet will be upset and the component will not be supplied with oil in the optimum manner.